In analyzing the timing of an internal combustion engine, determination is made of the timing angle or number of degrees between the time a piston in a selected cylinder reaches its top-dead-center position and the time that combustion takes place in that cylinder. Automobile manufacturers commonly specify the number of degrees at a specific engine speed as a compromise to numerous considerations which must be taken into account. Speed of the engine can be determined by knowing the frequency of top-dead-center events.
The occurrence of top dead center is identified in the following manner. An internal combustion engine includes a wheel mounted on the end of the engine crankshaft so as to rotate therewith. A notch or other marking means is located in the periphery of the wheel. A receptacle for a magnetic probe is attached to the engine block and is so located that the marking means on the rotating wheel will pass the receptacle a known number of degrees of crankshaft rotation after the number one (or other selected) cylinder has reached its top-dead-center position.
Pat. No. 4,373,384, assigned to the assignee of this application, describes a timing/tach meter for diesel engines. Such meter senses the combustion event by light occurring during combustion. The glow plug for the prechamber associated with the selected cylinder is removed, and a luminosity probe forming part of the timing meter is inserted in its place. The luminosity probe includes a sensor which responds to light produced during combustion and generates a luminosity signal. In the patent, circuitry compares the instant of occurrence of a magnetic event with the instant of occurrence of a luminosity event to calculate the timing angle. This meter is not used, as such, to time a spark-ignited engine. A different timing meter which includes structure to identify top-dead-center events, or stroboscopic timing light is required for that purpose. But, the combustion event in a spark-ignited engine is sensed not by the occurrence of light, but rather by pulses of electric current flowing through the distributor conductor to a spark plug during combustion. Engine speed can be determined by the frequency of such pulses.
In other words, it has been heretofore necessary to employ two timing and/or speed measuring devices, one for analyzing the timing and/or speed in a diesel engine and another for analyzing the timing and/or speed in a spark-ignited engine.